In a typical cellular radio communication system (wireless communication system), an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective antenna or antenna system. The antennas in the cells are in turn coupled to a controller, which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (MSC) and/or a packet data serving node (PDSN) for instance. These (and possibly other) elements function collectively to form a Radio Access Network (RAN) of the wireless communication system. The switch or gateway may then be coupled with a transport network, such as the public switched telephone network (PSTN) or a packet-switched network (e.g., the Internet).
Depending on the specific underlying technologies and architecture of a given wireless communication system, the RAN elements may take different forms. In a code division multiple access (CDMA) system configured to operate according to IS-2000 and IS-856 standards, for example, the antenna system is referred to as a base transceiver system (BTS), and is usually under the control of a base station controller (BSC). In a universal mobile telecommunications system (UMTS) configured to operate according to Long-Term Evolution (LTE) standards, the antenna system is usually referred to as a NodeB or an eNodeB, and the entity that typically coordinates functionality between multiple eNodeBs is usually referred to as a mobility management entity (MME). Other architectures and operational configurations of a RAN are possible as well.
A subscriber (or user) may access the wireless communication system for communication services via a wireless communication device (WCD), such as a cellular telephone, “smart” phone, pager, or wirelessly equipped portable computer, for instance. In a CDMA system the WCD may be referred to as an access terminal (AT) or a mobile station. In an LTE system the WCD may be referred to as user equipment (UE). When an AT or UE is positioned in a cell, it communicates via an RF air interface with the BTS or eNodeB antenna of the cell. Consequently, a communication path or “channel” is established between the AT or UE and the transport network, via the air interface, the BTS or eNodeB, the BSC or MME, and the switch or gateway.
As the demand for wireless communications has grown, the volume of call traffic in most cell sites has correspondingly increased. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of cell-sectors, each defined respectively by radiation patterns from directional antenna components of the respective BTS or eNodeB, or by respective antennas.
Functionally, a BTS of a cell or an eNodeB may be referred to as a “base station.” The actual physical configuration of a base station can range from an integrated BTS-BSC or eNodeB unit to a distributed deployment of multiple BTSs under a single BSC, or multiple eNodeBs under a single MME. Regardless of whether it is configured to support one cell, multiple cells, or multiple sectors, a base station is typically deployed to provide coverage over a geographical area on a scale of a few to several square miles and for tens to hundreds to several thousands (or more) of subscribers at any one time.
In some areas, a wireless service provider may provide service via multiple wireless networks that operate according to different air interface protocols. For example, a first wireless network that operates according to a first air interface protocol (e.g., LTE) may be used for packet-based data communications, and a second wireless network that operates according to a second air interface protocol (e.g., CDMA) may be used for circuit-switched voice communications. A WCD may be able to operate under either of the first and second wireless networks, and may further be capable of handing off between them.
In some cases, a WCD may be handed off from a first wireless network to a second wireless network (or may otherwise transition from the first to the second wireless network) as part of a circuit-switched fallback (CSFB) process. The CSFB process may be performed, for example, to transition a WCD from a first wireless network (e.g., an LTE network) that is used for packet-data communications to a second wireless network (e.g., a CDMA network) that is used for circuit-switched communications so that the WCD can originate or receive a circuit-switched voice call.